Humidity plays a very major role in various industrial and commercial applications. Monitoring and controlling humidity is of great importance for the reliable operation of various systems. For example, solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors are fabricated utilizing semiconductor processes. Humidity sensors represent but one class of semiconductor-based sensors finding a useful industrial application. Modern manufacturing processes, for example, generally require measurement of moisture contents corresponding to dew points between −40° C. and 180° C., or a relative humidity between 1% and 100%. There is also a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres.
Humidity can be measured by a number of techniques. In a semiconductor-based system, for example, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are variations in resistance and the change in dielectric constant, which can be respectively translated into a resistance change and a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result includes erroneous readings, among other problems.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
A conventional capacitive humidity sensor, in general, can include a semiconductor substrate, and a pair of electrodes, which are formed on a surface of the semiconductor substrate and face each other across a particular distance. A humidity-sensitive film may also be placed between the electrodes and formed on a surface of the semiconductor substrate. The capacitance of the film changes in response to humidity. The sensor detects humidity by detecting changes in capacitance between the pair of electrodes in response to variations in the surrounding humidity. Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure, along with a layer or coating of a dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and attaining equilibrium in a short period of time. The response offset and slope for the integrated relative humidity sensor can be set to particular values in order to achieve a desired value of accuracy for the sensor.
FIG. 1 illustrates a “prior art” charge balancing circuit 100 of a humidity sensor for transforming measurements of relative humidity into a linear voltage. The high impedance capacitive nature of the humidity sensor can be readily handled by control of charge. FIG. 1 includes fixed capacitors C0, C1, C2, C3, and Cref that are designed to be insensitive to humidity and that can be fabricated at the same time and from the same materials. A humidity sensitive capacitor Cx can be designed to be sensitive to humidity and is fabricated at a different time and from different materials than the aforementioned capacitors. A switching matrix 120 varies the wiring scheme for capacitors: Cx, C0, and Cref utilizing two-phase, non-overlapping, dual polarity clocks, as can be provided by clock generator 110. Inverters A1, A2, and A3, and capacitor C1, and a pair of associated transmission gates 130 and 140 form a high gain comparator. The capacitor C2 and its pair of associated transmission gates 150 and 160 are the switched capacitor equivalent of a resistor which can be coupled with amplifier A4 and feedback capacitor C3 from an integrator. The capacitive values of the sensing capacitor Cx and the fixed capacitor C0 can be varied by laser trimming or by etching the sensing capacitor Cx to create voids in order to keep their values substantially equal.
FIGS. 2A and 2B illustrate “prior art” charge balancing circuit 200 and 250 during “Phase 1” and “Phase 2” operation respectively. In Phase 1 C0 is pulled up to Vcc and Cx is pulled down to GND and vice versa during Phase 2. Thus a periodic differential voltage can be created which is a function of the difference in capacitance values. The following equations mathematically describe the operation of the circuit 200 and 250. The charge at the summing node during Phase 1 and 2, can be calculated utilizing equations (1) and (2) respectively. The negative feedback results in Qs1 and Vs1 being substantially equal to Qs2 and Vs2. equation (3) mathematically describes the resulting transfer function for the complete circuit operation.Qs1=Cx*Vs1+C0*(Vs1−Vcc)+Cref*(Vs1−Vout)  (1)Qs2=Cx*(Vs2−Vcc)+C0*Vs2+Cref*Vs2  (2)Vout=Vcc*(Cx*(1+α*RH)/Cref)−Vcc*(C0/Cref)  (3)
In a majority of prior art humidity sensors the humidity sensitive capacitor Cx can be laser trimmed for offset adjustment and a photo mask layer of the reference capacitor C0 can be varied for slope adjustment. The laser trimming of humidity sensitive capacitor Cx for offset adjustment can introduce a reliability issue, due to exposure of the trimming site of the humidity sensitive capacitor to various application conditions. Also, the slope adjustment by variation of photo mask layer is costly and time consuming.
Based on the foregoing it is believed that a need exists for an improved methods and systems for adjusting characteristics of the relative humidity sensors in order to provide a more accurate measurement of humidity as will be disclosed in further detail herein.